Interpretive Summary: The semi-arid Central Great Plains region (CGPR) of the United States is known as the breadbasket of the country. This is because the greatest portion of the dryland farmland in that region is planted to hard red and hard white winter wheat (bread wheats). Because of precipitation limitations much of the winter wheat in the region is still planted in a two year rotation of winter wheat summer fallow (WF). The 14 month summer fallow period is used as a cultural practice to store water in the soil profile for use by the wheat in the subsequent year. The idea is to reduce the risk of crop failure due to drought. Traditionally weeds were controlled during the fallow year with tillage. As herbicides became available during the past 4 decades farmers and researchers found that no-till practices could be used instead of tillage to control weeds. With no-till/herbicides greater amounts of soil water storage resulted in greater wheat yields than with tilled systems. In this 40 year study we compare the long term effects of four fallow management practices: no-tillage, reduce tillage traditional shallow sweep tillage and moldboard plowing on soil quality. The soil quality parameters measured were soil organic carbon (SOC), Particulate organic matter (POM) and soil aggregation. With the no-till and reduce till managed plots we found greater soil quality at the soil surface as reflected by SOC, POM and soil aggregation. However, we also found that the moldboard plowed system forced a SOC redistribution by simply burying surface crop residues deeper in the soil.

Technical Abstract:
Conventional tillage (CT) winter wheat (Tritucum aestivum L.) with summer fallow (WF) is the predominant cropping system in the central Great Plains. We investigated the effect of 39 yr of different tillage intensities, conventional tillage (CT); moldboard plow (MP); no-tillage (NT); and reduced tillage (RT) on soil organic carbon (SOC), organic carbon fractions, particulate organic matter (POM), and wet aggregate-size distribution. A long-term tillage study was initiated in 1967 near Akron, CO on a silt loam. In 2006, soil samples were collected from the 0- to 5-, 5- to 10-, and 10- to 20-cm depths in WF. No-tillage and RT significantly increased SOC, at 0- to 20-cm depth, by an average of 25% compared with CT and MP with two calculation approaches: fixed depth and equivalent mass basis. Tillage significantly impacted POM-C at all depths studied and at 0- to 5- and 10- to 20-cm depth for mineral-associated organic matter C (MAOM-C). Changes in POM-C were more pronounced than the changes in MAOM-C. The MAOM-C accounted for an average of 78% of SOC, at the 0- to 5-cm depth, where POM-C accounted for 22% of SOC. Redistribution and stratification of SOC, POM, POM-C, and MAOM-C were observed especially with MP. The proportion of macroaggregate associated with NT and RT were higher than those associated with CT and MP with a concomitant increase in the proportion of microaggregates with CT and MP relative to NT and RT. Particulate organic matter C and MAOM-C exhibited a significant and positive relationship with SOC that was influenced by the soil depth. Over all, tillage elimination or reduction in tillage intensity/frequency associated with WF had a positive influence on soil aggregation and SOC dynamics in the central Great Plains that exhibit low SOC input. [GRACEnet Publication].